Apparatus For Bonding Wafers and an Optically-Transparent Thin Film Made from the Same

ABSTRACT

A novel apparatus for bonding of two polished substrates includes a plasma source in a ultra-high vacuum (UHV) chamber and a wafer-guiding element to control and guide wafers in the UHV chamber, where after a plasma activation process the wafers are guided and pressed against each other to form a covalent bond between wafer surfaces. The plasma activation process involves deposition of mono-layer or sub-monolayer metallic atom on the surface of substrates. After deposition of metallic layers, a high-force actuation presses the wafers and forms a covalent bond between the wafers. Then, the bonded wafer pair is ion-sliced or thinned to form single crystalline optical thin film. An annealing process oxidizes the deposited metallic layers and produces optically-transparent single crystalline thin film. An optical waveguide may be fabricated by this thin film while utilizing an electro-optic effect to produce optical modulators and other photonic devices.

FIELD OF THE INVENTION

The present invention generally relates to an apparatus for bonding twopolished surfaces. The present invention is also related to productionof functional thin-film substrates from single crystalline or amorphousmaterials for optical waveguide applications, opto-electronic devices,acoustic devices, RF devices, and other electronic and opto-electronicdevices by utilizing the apparatus. More specifically, the apparatusrelates to production of thin film substrates of single crystallinepiezo-electric, electro-optic, semiconductor, or scintillator crystals.Furthermore, the apparatus also relates to production of thin filmsubstrates of lithium niobate, lithium tantalate, and similarelectro-optic crystals. Moreover, the apparatus also relates to abonding method to produce such thin film crystal substrates. Inaddition, the apparatus may also be utilized for bonding of thesecrystals or other crystals to other substrates, such as silicon,germanium, quartz, GaAs, InP, GaN, and other semiconductor or opticalmaterials. Furthermore, the apparatus also relates to bonding forwafer-level packaging of electronic, photonic, and MEMS devices.

BACKGROUND OF THE INVENTION

In various modern electronic or photonic applications,single-crystalline thin film materials are widely utilized in order toenable manufacturing of functional electronic or photonic devices. Forexample, thin films of electro-optic materials are needed to makeoptical modulators, electro-optic devices, acousto-optic devices, andnonlinear optical devices. Bonded thin film of semiconductors areparticularly useful for fabrication of high-speed or high-powerelectronic devices. Furthermore, high resolution x-ray detectors oftenrequire bonded thin films of scintillator crystals. A wafer-bonding stepis necessary to make the thin film layer, which is particularly usefulin manufacturing of various high-speed, high-power, x-ray, and/orphotonic devices.

Several conventional methods exist for semiconductor and dielectricsubstrates wafer bonding involving direct or indirect bonding. Inindirect wafer bonding techniques, an intermediate layer, such as a glueor some adhesive, is used to attach two wafers. On the other hand, indirect wafer bonding techniques, no adhesive or intermediate layer isneeded. For many electronic and photonic applications, indirect waferbonding methods are not desirable due to high-temperature processingsteps during fabrication of devices that are not compatible withadhesive bonding. Furthermore, adhesive bonding methods are not reliableand may not be transparent to optical wavelengths, thus making theconventional indirect wafer bonding techniques generally undesirable foroptoelectronics manufacturing.

Direct wafer bonding does not have an adhesive or intermediate layerbetween the wafers. The direct wafer bonding techniques can be dividedto high temperature, low temperature, or room temperature methods. Inhigh temperature methods, wafers need to be heated to a high temperatureto achieve wafer bonding. This method works for bonding similarmaterials or materials that have exact same coefficient of thermalexpansion (CTE). If the materials have different CTE the bonding willcause significant stress in the substrates and is not practical to usethis method for bonding dissimilar materials. Similarly, low temperaturewafer bonding is not appropriate for bonding of materials with differentCTE.

In recent years, room temperature wafer direct bonding has beenintroduced for industrial applications. This new method uses anultra-high vacuum (UHV) environment. In general, there are two methodsfor room temperature wafer bonding. Surface Activated Bonding (SAB) andAtomic Diffusion Bonding (ADB) are two methods that have been developedto achieve wafer bonding process (T. Shimatsu, M Uomoto, “Atomicdiffusion bonding of wafers”, J. Vac. Sci. Tehnol. B 28(4), pp 706-714,2010, E. Higurashi, T Suga, “Review of Low-Temperature BondingTechnologies and Their Application in Optoelectronic Devices”,Electronic and Communications in Japan, Vol. 99, No 3, pp 63-71, 2016).

In SAB, the surface of the wafers is plasma-treated in a UHV chamberwhere the native oxide layer on a surface is removed and dangling bondsare formed on the surface. These dangling bonds then allow wafer bondingto be formed in the UHV chamber to achieve bonded substrates. Theactivated surfaces are brought into contact in UHV chamber where thedangling bond can form a covalent bond between two substrates. Thecovalent bond is very strong. Hence the bond strength for these bondedsubstrates is very strong. The SAB method is more useful for bonding ofmetals or semiconductors.

For lithium niobate or other single-crystalline oxide materials, the SABmethod may not work well because no dangling bond can be formed on thesurface by the plasma treatment when the material for targeted bondingis an oxide material. For bonding of single-crystalline oxide materials,ADB method is employed in which a mono-layer of metals is deposited onthe surface of the crystal in UHV chamber. This monolayer metal need tobe deposited in a UHV chamber in order to prevent oxidation of themetal. The metal creates dangling bonds on the wafer surface where thedangling bonds can interact and form a covalent wafer bonding similar tothe covalent bonding formed by the SAB method.

The ADB might be compared to an adhesive-based bonding method where themono-layer metallic layer is the adhesive between the two substrates.However, the thickness of the deposited metal layer is a mono-layer or asub-monolayer. The small amount of deposited material can easily beoxidized or diffused into the substrate, and hence it will not act as ametallic layer. Because the thickness of this layer is very thin and isoften less than a mono layer, it will not interfere with electronic orphotonic properties of devices fabricated on such substrates. Becausethe amount of deposited material is a sub-monolayer, it hasinsignificant or no effect on device performance. Therefore, the ADBmethod can also be classified as a direct bonding method.

For optical waveguide and opto-electronic device fabrication andmanufacturing, a wafer bonding is needed between opticalsingle-crystalline materials and a substrate. The SAB method does notwork well in this case because the dangling bond does not exist in anoxide material. The ADB method is generally more suitable formanufacturing applications that require bonding of single crystallineoxides. A thin layer of oxide material will then be formed after bondinga single-crystalline material to a second substrate using crystal ionslicing or thinning. The mono-layer or the sub-mono layer metal layerdeposited during bonding may still cause problems because a mono-layermaterial can still absorb light, which is not desirable in an opticalwaveguide or opto-electronic device applications. However, an annealingstep can be used to agitate the mono-layer metal to be oxidized anddiffused into the single-crystalline material, which ensures that a freeelectron absorption caused by metallic state is eliminated and a bondedthin-film layer is transparent to optical signals.

It may be desirable to devise a novel apparatus and a correspondingmanufacturing process to accommodate wafer-bonding steps and fabricatethin films of various electronic or photonic materials. In particular,it may be desirable to devise a novel polished surface-bonding apparatusthat provides bonding of two different single-crystalline or amorphousmaterials at room temperature using ADB or SAB methods. Furthermore, itmay also be desirable to devise the polished surface-bonding apparatusto utilize uniquely-structured hinges to place wafers inside anultra-high vacuum (UHV) environment and a plasma source for achievingsurface activation.

In addition, it may also be desirable to devise a novel polishedsurface-bonding apparatus that exhibits a streamlined and low costwafer-bonding procedure with a high bonding throughput. Moreover, it mayalso be desirable to devise a novel device manufacturing method toproduce low-loss optical waveguide devices with high productivity byutilizing ADB and annealing techniques.

SUMMARY

Summary and Abstract summarize some aspects of the present invention.Simplifications or omissions may have been made to avoid obscuring thepurpose of the Summary or the Abstract. These simplifications oromissions are not intended to limit the scope of the present invention.

In one embodiment of the invention, a novel mechanical apparatus isintroduced for manipulation and bonding of the wafers in a UHVenvironment. The novel apparatus comprises a plasma deposition source, ahinge element where the wafers are placed on the hinge element to beprocessed in the UHV chamber, and a high-force actuation element toachieve wafer bonding. The hinge element controls or guides the wafersin three positions: loading-unloading, pre-deposition, deposition, andbonding. The plasma deposition source deposits a mono layer of materialson wafer surfaces, or removes a native oxide layer on the wafer surfacesduring pre-deposition and deposition process steps. A high-forceactuation element is then used to press the wafers inside the ultra-highvacuum chamber to achieve wafer bonding.

In another embodiment of the invention, a method for producing a thinoptically transparent layer or a slab optical waveguide is disclosed.This method comprises the steps of: bonding a single-crystallinematerial to a cladding material using the apparatus discussed above toachieve atomic diffusion bonding; thinning or crystal ion-slicing thesingle crystalline material to several hundred nanometers or severalmicrons; annealing a bonded and thinned single-crystalline substrate athigh temperatures to diffuse a bonding mono-layer metal layer andachieve low-loss optical thin film layers that can be used for waveguideor other opto-electronic applications.

Yet in another embodiment of the invention, the novel bonding apparatusdisclosed herein may be used for a variety of other electronicapplications. For example, for acousto-optic devices, a transducer istypically bonded to generate acoustic waves in an acousto-optic crystal.A piezo-electric material, such as lithium niobate or lithium tantalate,is usually bonded to acousto-optic materials, such as germanium ortellurium oxide. The acoustic wave may be absorbed if an intermediatelayer exists between the piezo-electric material and the acousto-opticmedium. Preferably, ADB is utilized to directly bond the transducer tothe acousto-optic crystal.

Yet in another embodiment of the invention, the novel bonding apparatusis utilized in high-power electronics that generate high amounts ofheat, wherein the bonding of highly thermally-conductive materials isdesirable for efficient heat dissipation. For example, the novel bondingapparatus is advantageous in manufacturing of high-power electronicdevices, such as a power amplifier, where a highly thermally-conductivesubstrate is bonded to the power amplifier to achieve heat dissipation.SAB or ADB bonding methods may be utilized to bond highly-thermallyconductive materials to an integrated circuit or another electroniccomponent-holding substrate and remove heat from a high-power electronicdevice. Furthermore, other electronic or photonic applications involvingthermal management, x-ray detectors, or other photo-electronicapplications that require bonding of polished surfaces may utilize thenovel bonding apparatus, in accordance with an embodiment of theinvention.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A-1D show a plurality of schematics for a vacuum chamber designfor wafer bonding with surface activation and related process steps forbonding of semiconductor, oxide, or metallic material in a UHV chamber,wherein a frame is structured around the UHV chamber to withstand highpressure or impact forces experienced during the bonding step, and alsoshow a hinge element utilized to manipulate wafers inside an ultra-highvacuum chamber, in accordance with an embodiment of the invention.

FIG. 2A and FIG. 2B show the overall and the cross section of the vacuumchamber for bonding of semiconductor, dielectric, or metallic wafers, inaccordance with an embodiment of the invention.

FIG. 3A and FIG. 3B show open and closed positions of a hinge elementfor bonding of substrates in a UHV chamber, in accordance with anembodiment of the invention.

FIG. 4 shows a chucks holding element for the wafer bonding system thatincorporates a spring loading element and a stopper element that areutilized in conjunction with interchangeable chucks, in accordance withan embodiment of the invention.

FIG. 5 shows the wafer retaining element in which a spring-loaded pin isused to hold the wafers and the pin is pushed back during a bondingstep, wherein the wafer retaining element also incorporates a plateattached to the chuck that act as a shutter, in accordance with anembodiment of the invention.

FIG. 6 shows a shielding element of the wafer bonding system, inaccordance with an embodiment of the invention.

FIG. 7 shows a high-level view of a high-force actuation element thatincorporates a frame around a UHV chamber to withstand the high forcesexerted during the bonding step and the actuator is de-coupled from thevacuum chamber using bellows, in accordance with an embodiment of theinvention.

FIG. 8A shows a conceptual diagram of a chamber to bond wafers using SABmethod where an ion gun is used, in accordance with an embodiment of theinvention.

FIG. 8B shows a conceptual diagram of a chamber to bond larger-sizewafers in which two plasma activation sources are used to activate wafersurfaces, in accordance with an embodiment of the invention.

FIG. 9A and FIG. 9B show process steps for fabrication of opticalwaveguides using a novel wafer bonding apparatus, in accordance with anembodiment of the invention.

FIG. 10A-10C show alternate process steps for fabrication of opticalwaveguides using a novel wafer bonding apparatus, in accordance with anembodiment of the invention.

DETAILED DESCRIPTION

Specific embodiments of the invention will now be described in detailwith reference to the accompanying figures. Like elements in the variousfigures are denoted by like reference numerals for consistency.

In the following detailed description of embodiments of the invention,numerous specific details are set forth in order to provide a morethorough understanding of the invention. However, it will be apparent toone of ordinary skill in the art that the invention may be practicedwithout these specific details. In other instances, well-known featureshave not been described in detail to avoid unnecessarily complicatingthe description.

The detailed description is presented largely in terms of procedures,logic blocks, processing, and/or other symbolic representations thatdirectly or indirectly resemble a wafer bonding tool, a method ofbonding semiconductor, dielectric, or metallic surfaces, and/or a methodof low-loss dielectric optical waveguide fabrication or otheropto-electronic device applications as described in various embodimentsof the invention. These process descriptions and representations are themeans used by those experienced or skilled in the art to mosteffectively convey the substance of their work to others skilled in theart.

Reference herein to “one embodiment” or “an embodiment” means that aparticular feature, structure, or characteristic described in connectionwith the embodiment can be included in at least one embodiment of theinvention. The appearances of the phrase “in one embodiment” in variousplaces in the specification are not necessarily all referring to thesame embodiment. Furthermore, separate or alternative embodiments arenot necessarily mutually exclusive of other embodiments. Moreover, theorder of blocks in process flowcharts or diagrams representing one ormore embodiments of the invention do not inherently indicate anyparticular order nor imply any limitations in the invention.

For the purpose of describing the invention, a term “wafer bonding” isgenerally defined as a processing method, in which two material surfacesare brought into contact and a covalent chemical bond is achievedbetween atoms on a surface of a first material and atoms on a surface ofa second material.

In one embodiment of the invention, a novel apparatus is disclosed forbonding of polished surfaces.

FIG. 2A shows a perspective diagram (200A) of the apparatus with thevacuum chamber for bonding of semiconductor, dielectric, or metallicwafers, while FIG. 2B shows a cross section (200B) of the vacuumchamber, in accordance with a preferred embodiment of the invention.Furthermore, FIGS. 1A-1D show a plurality of schematics for a vacuumchamber design for wafer bonding with surface activation and relatedprocess steps for bonding of semiconductor, oxide, or metallic materialin a UHV chamber, wherein a frame is structured around the UHV chamberto withstand high pressure or impact forces experienced during thebonding step, and also show a hinge element utilized to manipulatewafers inside an ultra-high vacuum chamber, in accordance with anembodiment of the invention.

In one example, the novel apparatus for bonding of polished surfacescomprises the following elements:

1. A high vacuum chamber (101) (e.g. a UHV chamber) in FIGS. 1A-1D,where all the components of the novel apparatus are attached to thisbody.

2. A hinge element (104) in FIGS. 1A-1D controlled with a DC motor forposition changes and guiding of wafers inside the chamber. The hingeelement (104) is attached to a linear feed-through unit, and dependingon the location of the shaft in the linear feed-through unit, the wafersare placed in different positions.

3. Two wafer chucks (105,107) placed on the hinge element (104) in FIGS.1A-1D, which is utilized to guide and control the wafers inside thevacuum chamber. The chucks have two or several pins (FIG. 5) that arepushed out in normal condition by using a spring and are pushed in whenthe wafers are brought into contact for wafer bonding process.

4. A plasma source (103) in FIGS. 1A-1D placed above the chamber andconfigured to activate wafer surfaces.

5. Several shields (FIG. 6) and a shutter (e.g. 102 in FIG. 1A, FIG. 1C,FIG. 1D, FIG. 5) attached to one of the wafer chucks. The shieldsprevent deposition from the plasma source to chamber walls and sensitiveinstruments, while also creating a gettering medium where thecontamination on the wafer surface is reduced after the pre-depositionprocess. The shield prevents the particles to reach the substrate duringpre-deposition process step.

6. A bottom feedthrough configured to operate the hinge element andattached to a stepper motor, which is controlled by a PLC unit.

7. A frame (114) in FIGS. 1A-1D that encloses the UHV chamber withpneumatic or hydraulic actuation element (109), wherein the actuationelements place a force on the chucks inside the vacuum chamber, andwherein the frame is used to relive the reaction force for the bondingof the wafers. The frame is connected with two shafts and via afeedthrough to the wafer chuck using a bellow element. The actuationelement is de-coupled from the high vacuum chamber.

8. Two feedthroughs configured to apply force on the wafers and twobellows that are utilized to roll in a shaft that is coupled to theframe assembly. The bellows allow the shaft to be moved into the chamberduring the wafer bonding step while the high bonding force istransmitted through the shaft to the frame.

9. Two high-force bond actuation elements (i.e. 109) in FIGS. 1A-1Dattached to the frame, which are used to push or pull the bonding shaftin and out of the vacuum chamber and are used to provide the bondactuation force.

10. A vacuum pump utilized to pump the system in combination with otherpump(s).

11. One or more vacuum gauges utilized to monitor the pressure ofchamber.

12. Electronic control units that operate the vacuum pump, pressuregauges, and the manipulators.

13. A DC or RF plasma source power supply unit.

According to this embodiment of the invention, the apparatus for waferbonding achieves the process for the wafer bonding in by the followingsteps:

1. The wafer chucks are placed inside the hinge element and are held inplace with spring loaded elements (i.e. 401, 403, 405 in FIG. 4). Twowafers (i.e. 111, 113 in FIG. 1A) that should be bonded are placed onthe two wafer chucks in the unload position, as shown in FIG. 1A. Thewafer chuck pins are pushed out with a spring element, as shown in FIG.5, and hence they retain the wafer in position as shown in FIG. 1A.

2. The hinge element (104) is then partially closed to pre-depositionposition, as shown in FIG. 1B, where the shutter that is attached on onewafer chuck (i.e. FIG. 5) prevents deposition of materials from theplasma source to the surface of the wafers.

3. The system is pumped down to low vacuum levels.

4. The plasma source is turned on for several seconds, as illustrated inFIG. 1B. During this process, either a metal layer is sputtered of thesurface of the plasma source target and is deposited on the shutter, orif the plasma source is a ion source, the surface of the shield isbombard with ions.

5. The hinge element (104) is moved to the deposition position, as shownin FIG. 1C. The angle and the position are selected such that veryuniform surface activation is achieved across the surface of the twowafers. The plasma source is turned on again and either a metal layer isdeposited on the wafer surfaces for ADB method or native oxide layer ofthe wafer surface is removed for SAB method. In any case after thisprocess dangling bonds are formed on the surface of the wafers.

6. The hinge element (104) is then moved to the bond position (FIG. 1D).

7. The bond actuator element (109) is activated and two arms push thewafers together, as illustrated in FIG. 1D and FIG. 7, in the bondposition with high forces. The high force of the bond actuators isrelieved through the frame that act as a counter-force element for highbonding force needed. After this steps the wafers are bonded together.

8. The hinge element (104) is moved to load-unload position (i.e. FIG.1A), and the bonded wafer pair is unloaded from the chamber.

Furthermore, in a preferred embodiment of the invention, the polishedsubstrates may be up to 4″ single side or double side polishedsemiconductor, dielectric, oxide single crystal or amorphous materialswith a thickness as small as several microns to several millimeters. Theultra-high vacuum chamber may be made from aluminum or stainless steeland can reach a base vacuum pressure in the range of 10{circumflex over( )}-9 ton to 10{circumflex over ( )}-6 ton. The sputtered material canbe highly reactive metals such as chromium, tungsten, titanium or othermaterials such as aluminum.

The thickness of the deposited metal may be sub-mono layer to severalhundred nanometers. The deposition method can be DC sputtering sourcefor ADB method. Alternatively, the plasma source may be an ion sourcethat bombard the wafer surfaces and removes native oxide layer for SABbonding methods. During the bonding step the applied force may be inrange that provide a pressure ranging from 0.1 Mpascal to 1000 Mpascal.

FIGS. 2A-2B show the overall design (200A) and the cross section (200B)of the bonding apparatus. FIG. 2A shows the vacuum chamber withpneumatic or hydraulic bond actuators, the frame, the ports for plasmaactivation source and ports for other element of the system. FIG. 2Bshows the cross section (200B) of the chamber where in addition to thevacuum chamber, actuators, frames and ports, the hinge element, waferchucks, bellows and high force actuation element details can be seen.

FIG. 3A shows the details of the hinge element of the bonding apparatusand the wafer chucks in an open position (300A), while FIG. 3B shows thedetails of the hinge element and the wafer chucks of the bondingapparatus in a closed position (300B). These illustrations show detailedexemplary connections of the wafer chucks, the stopping block to preventthe wafer chucks to get too close to each other, and the hinge elementconfigured to mobilize and position the manipulate the wafer chucks.

FIG. 4 shows the spring loading element (400) of the bonding apparatusutilized to hold the chucks connected to the hinge element. Inparticular, this illustration shows a spring-loaded chuck (401),interchangeable chucks (405), and an interlocking shield (403) betweenthe interchangeable chucks (405).

FIG. 5 shows the wafer-retaining element (500) of the bonding apparatuswhere spring-loaded pins (501) are used to hold the wafers on the twochucks. The spring-loaded pins (501) are pushed in as the chucks areraised to the bonding position during a bonding step. Additionally, thewafer-retaining element (500) also incorporates a plate (503) attachedto the chuck that act as a shutter during the bonding step.

FIG. 6 shows one or more additional shields (601A, 601B) on the bondingapparatus (600). The additional shields (601A, 601B) configured toprevent the materials to be deposited on chamber walls during apre-deposition process. At least in some cases, each shield (601A, 601B)also acts as a gettering media and absorb contaminations that exist inthe chamber and allow higher-quality bonding to be achieved.

FIG. 7 shows a cross-sectional diagram (700) of a high-force actuationelement in the bonding apparatus that incorporates a frame around a UHVchamber to withstand high forces exerted during the bonding step and anactuator which is de-coupled from vacuum chamber using bellows, inaccordance with an embodiment of the invention. Since very high forcesare needed during the bonding, the frame element allows the forces to berelieved and not excreted on the high vacuum chamber during the bondingstep.

FIG. 8A shows a conceptual diagram (800A) of a chamber to bond wafersusing SAB method where an ion gun is used, in accordance with anembodiment of the invention. In particular, this figure illustrates asystem design for bonding based on SAB method. The plasma source in thisdesign is an ion gun with a collimated ion sources. Because the ion beamis non-divergent and needs to cover the surface of wafers, the wafersare tilted upward. The ions hit the surface of the wafers at a glazingangle and remove a few monolayers of oxide layers from one or moresurfaces of the wafers. The wafers are then returned to horizontalposition and bonded similar to what is shown in FIG. 1.

FIG. 8B shows a conceptual diagram (800B) of a chamber to bondlarger-size wafers in which two plasma activation sources are used toactivate wafer surfaces, in accordance with an embodiment of theinvention. This illustration shows an alternative design where twosputtering sources or two ion guns are used where the system can be usedto bond materials with diameter exceeding 4″ diameter for example 6″ or8″ diameter wafers.

This particular embodiment discloses a few optical and electronicapplications where the apparatus for bonding of wafers may be used tomake these devices. In one embodiment of the invention, a method tofabricate a highly optically transparent thin film of single crystallineoptical materials is disclosed. In this method the thin film is formedby bonding a single crystalline or amorphous material to a substrateusing the wafer bonding method described. The resulting thin film haslow optical absorption loss and can confine the light in the verticaldirection. An optical circuit or an opto-electronic device will then bedefined on this thin film using methods that are well-known to a personof ordinary skill in the art to make various integrated optical devices.

As previously illustrated and shown in FIGS. 1A-8B, in the preferredembodiment of the invention, the novel apparatus for bonding of twopolished substrates includes a plasma source in a ultra-high vacuum(UHV) chamber and a wafer-guiding element to control and guide wafers inthe UHV chamber, where after a plasma activation process the wafers areguided and pressed against each other to form a covalent bond betweenwafer surfaces. The plasma activation process involves deposition ofmono-layer or sub-monolayer metallic atom on the surface of substrates.After deposition of metallic layers, a high-force actuation presses thewafers and forms a covalent bond between the wafers.

Then, the bonded wafer pair is ion-sliced or thinned to form singlecrystalline optical thin film. An annealing process oxidizes thedeposited metallic layers and produces optically-transparent singlecrystalline thin film. An optical waveguide may be fabricated by thisthin film while utilizing an electro-optic effect to produce opticalmodulators and other photonic devices. An optical cavity may be formedusing the thin film of single crystalline optical material producedusing the disclosed method where the single crystalline optical materialis bonded on a distributed Bragg mirror (DBR) intermediate layer and atop DBR structure is deposited on the single crystalline thin filmoptical material. An optical cavity is produced where the lightintensity can be switched using electro-optic effect by adjusting therefractive index of the single crystalline optical material and byapplying a voltage to the device using two electrodes.

FIG. 9A and FIG. 9B show process steps for fabrication of opticalwaveguides using the novel wafer bonding apparatus described previouslyin association with FIGS. 1A-8B, in accordance with an embodiment of theinvention. According to this embodiment, the process steps for producinga thin film by utilizing the novel wafer bonding apparatus are shown inFIG. 9A-9B. In one example, the method comprises the following steps:

1. Two smooth substrate surfaces (i.e. 901, 903) are produced bypolishing or polishing followed by a high temperature annealing step.

2. Metallic layer(s) are deposited on the smoothed surfaces of a coresubstrate (903) and a handle substrate (901), typically with a plasmasource. In some cases, an intermediate layer (902) is formed usingnanocrystalline metallic atoms. Then, using the specialized waferbonding apparatus previously described, the core substrate (903) withthe material for a thin film layer is bonded to the handle substrate(901). A bond is formed based on atomic diffusion bond process.

3. The core substrate (903), which is now attached to the handlesubstrate (901), is then thinned down to several hundred nanometers toseveral microns using grinding or lapping methods as shown in FIG. 9B.After this step, a thin film (904) made of the core substrate materialis formed.

4. At the final stage, a slab waveguide is annealed at high temperatureof more than 400 degrees. During this process, the metallic adhesionlayer for bonding is diffused into the core substrate (903) or to thehandle substrate (901), and a low optical loss thin film (i.e. 904) madeof the core substrate material is achieved.

According to this embodiment of the invention, the method for productionof slab waveguide allows to achieve low-loss optical thin films bybonding materials with different CTEs by annealing at high temperaturesafter the bonded substrate pairs are thinned down to several microns.

The final product of the manufacturing process is a thin film of asingle crystalline material as the core layer and a single crystallineor amorphous material as the cladding layer.

In addition, the structure resulting from the novel method ofmanufacturing the slab waveguide, in accordance with an embodiment ofthe invention, achieves low-loss thin films. After the thin film isformed, several different methods may be used to make optical waveguidesor other opto-electronic devices known to a person skilled in the art.

FIG. 10A-10C show alternate process steps for fabrication of opticalwaveguides using the novel wafer bonding apparatus described previouslyin association with FIGS. 1A˜8B, in accordance with an embodiment of theinvention. This alternative process steps for optical waveguidefabrication for making highly optically-transparent thin films of singlecrystalline materials are illustrated in FIGS. 10A˜10C. The methodcomprises the following steps:

1. Two very smooth substrate surfaces (i.e. 1001, 1003) are produced bypolishing or polishing followed by a high-temperature annealing step

2. A core substrate (1003) is ion-implanted with hydrogen or helium at ahigh dose. In a preferred example, the high dose is defined as a doseranging from approximately 1e16/cmA2 to 8e16/cmA2. In another example,the high dose range may be higher or lower than the preferred example.

3. The core substrate (1003) is then bonded to a handle substrate (1001)using nanocrystalline metallic atoms. Consequently, a bond is formedbased on atomic diffusion bond effect. The handle substrate (1001) mayoptionally include an intermediate layer (1002), as shown in FIGS.10A-10C.

4. The core substrate (1003), which is attached to the compositesubstrate (i.e. 1001 and/or 1002) is then thinned down to severalmicrons using grinding or lapping methods, as shown in FIG. 10B. Afterthis thinning process, the core substrate's thickness is reduced to lessthan 50 microns, and becomes a thin film made of the core substratematerial.

5. The resulting core substrate is then placed in an oven, and a thinlayer of core material is sliced at the position of hydrogen or heliumion-implanted sites (1004). A final thin film (1006) made of the corematerial is formed after this process.

6. At this final stage of the waveguide fabrication, the thin film(1006) is annealed at high temperature, typically at more than 400degrees. During this process, a metallic adhesion layer is diffused intothe core layer or the cladding layer, and a optically highly-transparentthin film material is achieved. Optionally, the sample may undergoslight to moderate polishing to reduce the surface roughness ofion-sliced film at the final stage of the waveguide fabrication.

Moreover, in one embodiment of the invention, the core substrate may belithium tantalate, lithium niobite, or other electro-optic crystals. Theinitial thickness of the core substrate (i.e. 903 or 1003) may bebetween 100 microns to one millimeter. The handle substrate may bequartz material that forms the cladding layer of a waveguide, a siliconsubstrate with silicon dioxide intermediate layer grown on its surfacethat forms the cladding layer for the slab waveguide, or another siliconsubstrate with other intermediate layers, such as Bragg grating or withno intermediate layer. Other intermediate layers may be formed bydeposition methods.

In one embodiment of the invention, the thickness of the handlesubstrate may be 100 microns to several millimeters. Furthermore, thethickness of intermediate layer may be 100 nm to 20 microns, dependingon the applications. A typical thickness is 1 micron. The intermediatelayer may be grown or deposited on the substrate. The conductivity ofsilicon handle substrate might be very small (less than 0.0001 ohm-cm)or very large (more than 5000 ohm-cm) depending on the applications. Asmoothening process may be used to reduce the surface roughness of thesubstrates prior to bonding step.

In one example, the surface roughness of the substrates may be 5angstrom to 0.1 angstrom after smoothening process and prior to bonding.An annealing process may be applied to achieve smoothening of thesurface of the substrates. The bonding process may deposit a mono-layeror sub-monolayer or several layers of metallic atoms on the substrate.The bonding pressure may vary between 0.1 Megapascal (MPa) to 1000Megapascal (MPa), depending on the type of surface utilized for bonding.The thickness of a thinned core or an ion-sliced substrate after thethinning process may be 0.1 microns to 10 microns. A typical thicknessis 300 nm. The annealing temperature for slab waveguide may be 500degrees to 700 degrees. A typical annealing temperature is 550 degreescentigrade for a duration of 48 hours.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only byformalized claims that will be presented in a correspondingnon-provisional application.

What is claimed is:
 1. A wafer-bonding apparatus for bonding two waferscomprising: a vacuum chamber; a hinge element controlled by an electricmotor for position changes and guiding of wafers inside the vacuumchamber; a first wafer chuck and a second wafer chuck attached to thehinge element to hold the two wafers separately prior to bonding,wherein the first wafer chuck, the second wafer chuck, and the hingeelement, when commanded by an apparatus operator, are configured to moveto a wafer-bonding position that applies a massive compression pressureon the two wafers with a bond actuator element to bond the two wafers;the bond actuator element with a pneumatic or hydraulic shaft configuredto be pushed into the vacuum chamber to provide the massive compressionpressure on the first wafer chuck and the second wafer chuck holding thetwo wafers during the wafer-bonding position; and a plasma sourcelocated above the vacuum chamber, wherein the plasma source isconfigured to be activated for several seconds in preparation of bondingthe two wafers.
 2. The wafer-bonding apparatus of claim 1, furthercomprising one or more shields and a shutter attached to at least one ofthe first wafer chuck and the second wafer chuck, wherein the one ormore shields and the shutter protect chamber walls and sensitiveinstruments in the vacuum chamber from accidental depositions from theplasma source and from accidental contaminations.
 3. The wafer-bondingapparatus of claim 1, further comprising a bottom feedthrough attachedto the electric motor, wherein the bottom feedthrough is configured tooperate the hinge element.
 4. The wafer-bonding apparatus of claim 1,further comprising a vacuum pump and vacuum and pressure gaugesoperatively connected to the vacuum chamber.
 5. The wafer-bondingapparatus of claim 1, further comprising an electronic control unitoperatively connected to the vacuum pump, the vacuum chamber, the hingeelement, the first wafer chuck, the second wafer chuck, the bondactuator element, and the plasma source to enable control of thewafer-bonding apparatus by the apparatus operator.
 6. The wafer-bondingapparatus of claim 1, wherein the plasma source is a DC sputteringsource to achieve atomic diffusion bonding.
 7. The wafer-bondingapparatus of claim 1, wherein the plasma source is an ion gun to achievesurface-activated bonding.
 8. The wafer-bonding apparatus of claim 1,wherein each of the first wafer chuck and the second wafer chuckutilizes one or more pins that are pushed out in a non-bonding position,and are pushed in when the two wafers are pressed against each other toform a bond in the wafer-bonding position by the bond actuator element.